Avtivated sludges from 42 sewage treatment plants (capacity over 1000 m(3)/d) located in the Mazovia region (PL) were studied for the title phenomenon and surveyed. The bulking took place in 26 plants at low air temps. during the autum, winter and spring periods as a result of an excessive growth of filamentous microorganisms. For removal of bulking sludge, an Al coagulant was used.